1. Technical Field of the Invention
The present invention relates to a two-stroke internal combustion engine which is suited for use in a portable power working machine, and in particular to a two-stroke internal combustion engine which is capable of minimizing the quantity of so-called blow-by or the quantity of air-fuel mixture to be discharged without being utilized for the combustion.
2. Description of the Related Art
An ordinary small air-cooled two-stroke internal combustion engine which is conventionally used in a portable power working machine, such as a chain saw or brush cutter, is constructed such that an ignition plug is disposed at the head portion of the cylinder, and an intake port, a scavenging port and an exhaust port, which are opened and closed by a piston, are provided at the trunk portion of the cylinder. According to this two-stroke internal combustion engine, one cycle of the engine is accomplished by two strokes of the piston without undergoing a stroke exclusively assigned to the intake or exhaust.
More specifically, in the ascending stroke of the piston, air-fuel mixture is introduced from the intake port into a crank chamber disposed below the piston. During the subsequent descending stroke, the air-fuel mixture is pre-compressed, and compressed gas mixture is then blown into a combustion actuating chamber (though it may be also called a combustion chamber, actuating chamber, cylinder chamber, etc., these chambers are generically referred to as combustion actuating chamber in the present specification) which is disposed above the piston, thereby enabling waste combustion gas to be discharged from the exhaust port. In other words, since the scavenging of the waste combustion gas is effected by making use of the gas flow of the air-fuel mixture, the unburned air-fuel mixture is more likely to be mixed into the combustion gas (exhaust gas), thereby increasing the quantity of so-called blow-by or the quantity of air-fuel mixture to be discharged into the atmospheric air without being utilized for the combustion. Because of this, two-stroke internal combustion engines are not only inferior in fuel consumption but also disadvantageous in that a large amount of poisonous components such as HC (unburned components in fuel) and CO (incomplete combustion components in fuel) are included in the exhaust gas when compared to four-stroke engines. Therefore, even if the two-stroke engine is small in capacity, the influence of these poisonous components on the environmental contamination should not be disregarded.
With a view to addressing these problems, various proposals on the two-stroke internal combustion engine have been suggested. The proposals have included the introduction of air into the combustion actuating chamber prior to the introduction of air-fuel mixture so as to scavenge the combustion gas (for example, Japanese Patent Unexamined Publications H9-125966 and H5-33657).
In such conventional air-preintroduction type two-stroke internal combustion engines, however, air inlet passageways or check valves are required to be separately installed, so the resulting engine may be complicated in structure, and inevitably result in a sharp increase in manufacturing cost.
Additionally, since air is caused to be excessively introduced into the combustion actuating chamber, the combustion stability may easily deteriorate, thus creating problems, such as difficulties in the engine setting. Accordingly, there exists a need in the art for a two-stroke engine which can overcome the aforementioned disadvantages associated with the conventional two-stroke engines.
An object of the present invention is to provide an air-preintroduction type two-stroke internal combustion engine which is capable of minimizing the quantity of so-called blow-by or the quantity of air-fuel mixture discharged without being utilized for combustion.
Another object of the present invention is to provide an air-preintroduction type two-stroke internal combustion engine with improved fuel consumption and engine power.
Yet another object of the present invention is to provide an air-preintroduction type two-stroke internal combustion engine capable of reducing the amount of poisonous components in the exhaust gas.
Still another object of the present invention is to reduce the manufacturing cost of an air-preintroduction type two-stroke internal combustion engine, without extensively altering its conventional structure.
These and other objects of the present invention, which will become apparent with reference to the disclosure herein, are accomplished by the two-stroke internal combustion engine according to the present invention, which includes one or more pairs of Schnxc3xcrle-type scavenging passageways, each allowing the combustion actuating chamber disposed above a piston to communicate with the crank chamber. The scavenging passageways are symmetrically provided on both sides of the longitudinal section that figuratively divides an exhaust port into two equal parts. A through-hole or a communication groove is provided at a skirt portion of the piston to thereby enable a scavenging inlet port provided at an upper end of the scavenging passageway to communicate with the crank chamber. In the descending stroke of the piston, combustion exhaust gas originating from the combustion actuating chamber is enabled to be introduced, via a scavenging outlet port provided at the downstream end of the scavenging passageway, into the scavenging passageway while closing the scavenging inlet port at the skirt portion of the piston. In synchronization with the descending stroke of the piston, the air-fuel mixture may be introduced from the crank chamber, via the through-hole or the communication groove, into the scavenging passageway. The combustion exhaust gas existing inside the scavenging passageway is designed to be introduced into the combustion actuating chamber prior to the introduction thereinto of the air-fuel mixture.
In a preferred embodiment, in the descending stroke of the piston, the scavenging outlet port is opened a moment after the exhaust port has been opened, which is followed by the opening of the scavenging inlet port through the through-hole or the communication groove.
Alternatively, the two-stroke internal combustion engine may include a pair of first Schnxc3xcrle-type scavenging passageways located close to the exhaust port and a pair of second Schnxc3xcrle-type scavenging passageways located away from the exhaust port, each passageway allowing the combustion actuating chamber, disposed above the piston, to communicate with the crank chamber. The scavenging passageways are symmetrically provided on both sides of the longitudinal section that figuratively divides the exhaust port into two equal parts. A through-hole or a communication groove is provided at a skirt portion of the piston to thereby enable a second scavenging inlet port, provided at an upper end of the second Schnxc3xcrle-type scavenging passageway, to communicate with the crank chamber. The upper end of the first scavenging passageway communicates via a throttle passageway with the second scavenging passageway. In the descending stroke of the piston, combustion exhaust gas originating from the combustion actuating chamber is introduced, via the first scavenging outlet port provided at the downstream end of the first scavenging passageway, into the first scavenging passageway, while the second scavenging inlet port is closed at the skirt portion of the piston. In synchronization with the descending stroke of the piston, the air-fuel mixture fed from the crank chamber is subsequently introduced into the second scavenging passageway via the through-hole or the communication groove, as well as through the second scavenging inlet port, and also into the first scavenging passageway through the throttle passageway. The combustion exhaust gas existing inside the first scavenging passageway is designed to be introduced into the combustion actuating chamber prior to the introduction thereinto of the air-fuel mixture.
In a preferred embodiment, in the descending stroke of the piston, the scavenging outlet port is opened a moment after the exhaust port has been opened, which is followed by the opening of the second scavenging inlet port via the through-hole or the communication groove and is followed by the opening of the second scavenging outlet port.
According to the preferred embodiments of two-stroke internal combustion engine of the present invention as described above, the air-fuel mixture supplied from the air-fuel mixture-generating means, e.g., a carburetor, is received and stored in the crank chamber in the ascending stroke of the piston.
When the air-fuel mixture inside the combustion actuating chamber, disposed above the piston, explodes and burns after being ignited, combustion gas is generated and the piston is pushed downward. In this descending stroke of the piston, the air-fuel mixture existing inside the crank chamber is compressed by the piston, and at the same time, the exhaust port is opened to permit the combustion exhaust gas to be discharged from the exhaust port.
As the piston further descends, the scavenging port, provided at the downstream end of each of the scavenging passageways, is opened. At this moment, the scavenging inlet port of each of the scavenging passageways is closed by the skirt portion of the piston. Since the combustion gas (combustion exhaust gas) pressure existing inside the combustion actuating chamber due to the aforementioned explosive burning is higher than the pressure of the air-fuel mixture inside the scavenging passageways, part of the combustion exhaust gas is permitted to blow down from the scavenging outlet port and hence is permitted to be introduced into and stored in the scavenging passageways.
As the piston still further descends, the scavenging inlet port is opened via the through-hole or the communication groove provided at the skirt portion of the piston, thereby allowing the air-fuel mixture that has been pre-compressed in the crank chamber to be introduced into the scavenging passageway. As a result, the combustion exhaust gas existing inside the scavenging passageway is pushed out by the air-fuel mixture introduced as mentioned above into the scavenging passageway, thereby enabling the combustion exhaust gas to be blown out from the scavenging outlet port toward the combustion actuating chamber. Due to the scavenging gas flow of the combustion exhaust gas thus blown out, the combustion exhaust gas existing inside the combustion actuating chamber is pushed out therefrom toward the exhaust port. Furthermore, following the combustion exhaust gas flow, the air-fuel mixture is also permitted to flow into the combustion actuating chamber, thereby completely forcing the combustion exhaust gas that has been introduced in advance into the combustion actuating chamber to flow toward the exhaust port.
In the preferred embodiment, the scavenging (by making use of the combustion exhaust gas) is performed by deliberately delaying the scavenging-initiating timing from the exhaust-initiating timing (the timing to open the exhaust port). In succession to this scavenging, a high-pressure fresh air (air-fuel mixture), which has been pre-compressed, is enabled to be introduced at a stretch into the combustion actuating chamber.
According to the conventional two-stroke internal combustion engine where the air-fuel mixture is employed as a scavenging flow, a fairly large quantity of air-fuel mixture is permitted to blow by, since the portion indicated by the hatched region (area) in FIG. 13 corresponds to the quantity of so-called blow-by of fresh air (air-fuel mixture).
According to the preferred embodiments of the two-stroke internal combustion engine of the present invention as described above, the quantity of blow-by of air-fuel mixture can be greatly reduced since the combustion exhaust gas is employed in place of the air-fuel mixture as a scavenging flow, which results in that the portion indicated by the hatched region (area) in FIG. 12 corresponds to the quantity of so-called blow-by of fresh air (air-fuel mixture). Therefore, according to the two-stroke internal combustion engine of the present invention, the quantity of so-called blow-by or the quantity of air-fuel mixture to be discharged without being utilized for the combustion can be reduced to a minimum, thus making it possible to improve the fuel consumption and engine power, and to minimize the amount of poisonous components in the exhaust gas.
Furthermore, the two-stroke internal combustion engine of the present invention can be manufactured by slightly modifying the piston (drilling a through-hole, etc. therein), so that the conventional engine structure would not be significantly altered. Therefore, it is possible to reduce the manufacturing cost of the engine to a minimum.
According to the prior art, it has been generally tried to narrow the timings of suction and exhaust (timing area) so as to reduce the amount of exhaust gas as a whole (the engine power would be inevitably lowered) in order to minimize the presence of poisonous substances in the exhaust gas (exhaust emission). According to the present invention, it is no longer absolutely required to reduce the amount of exhaust gas. Moreover, since air is no longer permitted to be introduced into the engine, it is now possible to prevent combustion stability from deteriorating and to facilitate the engine setting.
In accordance with the invention, the objects as described above have been met, and the need in the art for a two-stroke engine that has low amount of poisonous components in the exhaust gas, improved fuel consumption and combustion stability, has been satisfied.